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The chemical behavior of terminally tert -butylated polyolefins. A practical and efficient approach to imidazo[1,2- a ]pyridine-fused isoquinolines through the post-GBB transformation strategy. News 2.

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Dienes are widely used chelating ligands in organometallic chemistry. In some cases they serve as placeholder ligands, being removed during a catalytic cycle. For example, the cyclooctadiene "cod" ligands in bis cyclooctadiene nickel 0 are labile. In some cases, dienes are spectator ligands, remaining coordinated throughout a catalytic cycle and influencing the product distributions.

Chiral dienes have also been described. From Wikipedia, the free encyclopedia. This article is about organic chemicals. For the surname "Diene" used in West Africa, see Serer people. Noels Kirk-Othmer Encyclopedia of Chemical Technology.

Scheme 1: The first members of the [ n ]radialene series and retrosynthesis for [5]radialene 3. Another reason is probably the practical consideration that there are more and higher-yielding preparative routes to three-, four- and six-membered rings than to five-membered ones. Looking at possible precursors for 3 , it seems reasonable to start with a substrate that contains at least a complete five-membered carbocycle or — even better — a starting material that already displays all ten carbon atoms required in the product.

When starting with derivatives of type 6 it should be noted that these compounds can exist in the form of different diastereomers; for example — as shown in 5 — all five substituents could point in the same direction cis , cis , cis , cis -configuration.

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The Chemistry of Dienes and Polyenes, Volume 1.

Alternatively, one or more of these functionalized groups could be directed in opposite directions e. Whether this stereochemical difference will have an influence on the ultimate outcome of the multiple elimination process is an open question; but it is likely to influence the rate of this multi-step transformation, which will involve increasingly reactive unstable intermediates. As far as substrates are concerned that possess the complete set of necessary carbon atoms, the commercially available pentamethylcyclo-1,3-pentadiene 8 offers itself as the best starting material.

In the present publication we describe the synthesis of the two diastereomeric derivatives 5 and 7 bearing various functional groups see below , and describe the reaction of 8 with various bromination reagents.

Whenever possible we confirm the structures of intermediate compounds or byproducts by X-ray structural analysis. The pentaalcohol 16 is a known compound, having first been described by Tolbert and co-workers in in a short communication [7]. Although in a later contribution these authors reported on the all- cis geometry X-ray analysis of a derivative of this compound the pentamesylate, see below [8]. The experimental and spectroscopic details of the original synthesis were never fully published [9].

Cycloaddition of maleic anhydride MA to 12 subsequently afforded the endo -adduct 13 , which after esterification provided the diester 14 in excellent yield [7]. This bicyclic adduct was converted by ozonolysis followed by LAH reduction into the all- cis- derivative 15 , whose benzyl ether protection group was finally removed by catalytic hydrogenation over palladium on carbon [7,8]. Scheme 3: The preparation of derivatives of 16 better suited for nucleophilic substitution and elimination. Only in one case was a halogenated derivative obtained in poor yield: treatment of 16 with triphenylphosphine—bromine in carbon tetrachloride provided the tribromo ether derivative 19 as a minor component.

It is unclear at the present time whether the correspondingly configurated pentaalcohol was already present in the starting material 16 as a side-product, which had been overlooked in previous studies, or whether epimerization took place during the attempted conversion of the pentaalcohol to the pentabromide In all these experiments complex product mixtures resulted, the GC—MS analysis of which demonstrated that only partially halogenated derivatives of 16 had been produced; their chromatographic separation failed. Scheme 4: Preparation of the pentaacetate 21 from For the other two routes to precursors of [5]radialene 3 discussed here, the ten carbon atoms of the desired final product were available already at a very early stage of the synthesis.

The first route dates back to studies by Diels, Alder and co-workers [11,12] , carried out in the early s and 40s, which were taken up later by Le Goff and his students [13]. The only difference we noted between our results and those reported in the chemical literature concerns the ratio of 24 to One reason for this difference could be a pericyclic 1,5-hydrogen shift between the two valence isomers generating different ratios under different preparation and work-up conditions.

Another equilibration mechanism could involve a base-induced 1,5-hydrogen shift. Since the mechanism of formation of 24 and 25 was not the main concern of our investigations, we did not clarify these points. The differentiation between the symmetrical 24 and the non-symmetrical 25 by 1 H NMR spectroscopy is easy and was achieved by Le Goff. Still, the early workers [] did not distinguish between a possible meso -structure cis -orientation of allylic hydrogen atoms and the chiral C 2 -symmetrical configuration trans -orientation of hydrogen atoms of Although we were unable to separate the two components quantitatively by column chromatography on silica gel, the main isomer began to crystallize partially from the highly viscous hydrogenation mixture after standing for several months.

The crystals obtained were suitable for X-ray structural analysis, which showed that this isomer has the cis , cis , cis , trans- geometry, i. The structural assignment of the minor hydrogenation product as 27 , which could only be obtained in enriched form ratio 26 : 27 ca. Altogether there exist three hydrogenation products of 28 : the two shown 26 , 27 and also the all- cis -configurated pentaester, presumably the most highly strained of the three isomers. For symmetry reasons this compound should display two sharp signals in its 1 H NMR spectrum: one for the tertiary hydrogen atom at the ring and one for the methyl group of its five identical ester groups ratio It should not be difficult to identify these signals in the proton spectrum of a mixture of hydrogenation products because of their relative intensities.

Although a large number of signals can indeed be recognized in the corresponding chemical shift regions, none displays an extraordinary intensity. It is hence unlikely that the all- cis -pentaester is produced in significant amounts on hydrogenation.